I have mappress running on 2 sites and neither one of them is able to do this correctly. One install is wp3.5, the other was wp3.4 but is now on 3.5.1 and it’s broken on both sites.
When you make a new location and click the marker to edit the text in the visual editor, you will see blank text. You type in the stuff you want and hit save. When you click the marker again it is still blank. However clicking the html link and typing in there does save. You can also switch to the visual editor and see your text there, but if you save any changes while in the visual editor view it doesn’t save (but remember the original saved html text).